Position is used to define what is the role of that employee in the company, whether he is a developer, tester etc.Whereas Role defines what permissions that particular employee has when using ProcessMaker functionalities. As such position does not have a specific functionality as Role has in ProcessMaker.

One example use of position is while defining groups/departments, it helps in assigning users to proper groups/departments based on the role in the company.

Just to add to what Zainab said, you can make up any value for Position, since ProcessMaker doesn't use it. It just helps you remember what the user does in the organization. In contrast, Role is a set of permissions which defines what the user can do inside ProcessMaker.